The Global System for Mobile Communications (GSM) is the mobile communications standard that is used throughout Europe. FIG. 1 illustrates a GSM system 1 comprising a mobile unit 2 and a GSM base station 3. The mobile unit 2 has a transmitting part and a receiving part. The transmitting part of the mobile unit 2 comprises a microphone 10, an analog-to-digital (A/D) converter 11, a segmentation unit 12, a speech coder 13, a channel coder 14, an interleaver 15, a ciphering unit 16, a burst formatting unit 17, and a transmitter modulator 18. The receiving part of the mobile unit 2 comprises a receiver 40 for transmitting sound to the user, a digital-to-analog converter (D/A) 25, a speech decoder 24, a channel decoder 23, a de-interleaver 22, a de-cipherer 21, a Viterbi equalizer 20, and a receiver demodulator 19. An antenna 41 transmits signals both from the transmitter part and to the receiver part of mobile unit 2.
Base station 3 has a transmitting part and receiving part. The receiving part of base station 3 comprises a speech decoder 31, a channel decoder 30, a de-interleaver 29, a deciphering unit 28, a Viterbi equalizer 27, and a receiver demodulator 26. The transmitting part of base station 3 comprises a digital-to-digital (D/D) conversion unit 38 allowing for the input of data, a speech coder 37 for coding a voice signal, a channel coder 36, an interleaver 35, a ciphering unit 34, a burst formatting unit 33, and a transmitter modulator 32. An antenna 39 is used for both transmission by the transmitter part and reception by the receiving part of base station 3. Signals communicate between the mobile unit 2 and the base station 3 through a channel 4 which is typically an air interface.
Operation of the GSM system 1 precedes as follows for the case where the mobile unit 2 transmits and the base station 3 receives. A speaker speaks into microphone 10 producing an analog voice signal. The analog voice signal is applied to the A/D converter 11 resulting in a digitized speech signal. In GSM, 13 bits are used to quantize the signal into 8192 levels and the signal is sampled at an 8 kHz rate. The digitized speech waveform is then fed into the segmenter 12 which divides the speech signal into 20 ms segments. The segments are fed into the speech coder 13 for reduction of the bit rate. The speech coders defined for GSM today reduce the bit rate to 13 kbits/s. The next steps are channel coding and interleaving. The channel coder 14 adds error correcting and error detecting codes to the speech waveform. The interleaver 15 separates the consecutive bits of a message to protect against burst errors. The ciphering unit 16 adds bits to protect from eavesdropping. The burst formatting unit 17 formats bits into GSM burst frames and adds additional formatting bits (adds start and stop bits, flags, etc.) to each GSM burst frame. A typical GSM burst frame designed to fit within a Time Division Multiple Access (TDMA) slot may have, along with several formatting bits, 57 encrypted data bits followed by a 26 bit training sequence for the Viterbi equalizer followed by 57 encrypted data bits. The transmitter modulator 18 applies Gaussian Minimum Shift Keying (GMSK) modulation to the bit stream input producing a modulated radio frequency signal at its output suitable for transmission. The modulated radio frequency signal is transmitted via antenna 41 over channel 4 to antenna 39 of base station 3.
The receiver demodulator 26 receives the modulated radio frequency signal and demodulates the modulated radio frequency signal to a bit stream signal. The Viterbi equalizer 27 creates, based on the 26 bit training sequence, a mathematical model of the transmission channel 4, which in this case is an air interface, and calculates and outputs the most probable transmitted data. In the remaining signal processing chain, the de-ciphering unit 28 performs the inverse transformation performed by the ciphering unit 16, the de-interleaver 29 reverses the interleaving performed by interleaver 15, the channel decoder 30 reverses the channel coding of channel coder 14, and the speech decoder 31 recovers the original digital speech stream. Operation of the GSM system 1 precedes in a similar way in the situation where the base-station unit 3 transmits and the base station 2 receives.
The GMSK modulation scheme adopted for GSM is a digital modulation scheme which may carry data digitized speech. The GMSK modulator can be looked upon as a phase modulator. The carrier changes phase depending on the information bits sent to the modulator. GMSK includes the desirable feature of a constant envelope modulation within a burst. To get smooth curve shapes when changing the phase, the baseband signal is filtered with a Gaussian passband. With GMSK, a narrower bandwidth is obtained compared to ordinary MSK, but the price for this is less resistance against noise.
In a typical GMSK modulator, as known in the prior art, the transmitted data is first transformed into NRZ (Non-return-to-zero format). The NRZ data stream is then integrated and passed through a Gaussian filter. The pre-modulation Gaussian filter has narrow bandwidth and sharp cutoff properties which are required to suppress the high-frequency components of the integrated NRZ data stream. The Gaussian filtered signal is then used to modulate a carrier frequency producing in-phase (I) and quadrature (Q) components of a baseband signal. The I and Q components are then shifted to a carrier frequency W0 by multiplying the I and Q components by corresponding cos(nw0) and −sin(nw0) carriers and the two resulting flows are added to produce a carrier signal for transmission. A GMSK modulator has the following important characteristics: constant envelope, which-makes GMSK signals rather immune to power amplifier nonlinearities and less susceptible to fading, relatively narrow bandwidth and coherent detection capability.
Although GMSK is a current popular modulation technique, other modulation techniques will be used in future mobile communications systems. In particular, new systems like Enhanced Data Rate for GSM Evolution (EDGE) or Code Division Multiple Access (CDMA) will use linear modulation, such as M-PSK modulation. Linear modulation differs from GMSK modulation in that GMSK modulation has a constant envelope, whereas in linear modulation the amplitude of the modulated signal can vary in amplitude from a high amplitude to a low amplitude. Compared to present standards, like GSM, the modulation depth of a linear system may be quite large. The modulation depth is the difference in the modulated waveform between the lowest amplitude and the highest amplitude. For example, in the EDGE case, using 3π/8 8-PSK modulation, the modulation depth is about 17 dB.
In demodulation of a GMSK signal, it is possible to sample the GMSK signal at random times because the GMSK modulated waveform has a constant envelope. Thus, when the radio signal is de-modulated and filtered, it is possible to feed the samples at a symbol rate directly into a Viterbi equalizer. This simplifies the receiver considerably and reduces the required number of operations.
This simplified approach becomes more difficult with linear modulation such as, for example, EDGE using 3π/8 8-PSK. modulation, which typically has a modulation depth and consequently a variation in amplitude. One possible solution is to use over-sampling and digital post-processing of the signal to successfully demodulate a linear modulated waveform having a modulation depth. Although this approach is feasible, it typically requires complex signal processing and large memory.
In view of the foregoing, it would be desirable to provide a technique for demodulating a linear modulated waveform which overcomes the above-described inadequacies and shortcomings. More particularly, it would be desirable to provide a technique for demodulating a linear modulated waveform having a modulation depth in an efficient and cost effective manner.